The objectives of this project are to (1) determine if genes present on plasmid DNA of Borrelia burgdorferi, the causative agent of Lyme disease, control infectivity, (2) characterize these plasmids, (3) clone the infectivity genes and express those components that promote borrelia infection in mammals, and (4) determine immunogenic properties of components responsible for infection. The reduction in the number of detectable plasmids with the loss of infectivity suggests that gene(s), encoding for components related to infectivity, may be present on one or more of these extrachromosomal elements. A 7.6 kilobase (kb) pair circular plasmid has been identified as a strong candidate for regulating infection. Various restriction fragments from this plasmid have been cloned and are currently being used as probes for identifying infectious strains. These probes will also be useful in demonstrating the true fate of the 7.6 kb plasmid, which appears to be lost from the spirochete as a result of serial in vitro cultivation. Investigations concerning antigenic variation of B. burgdorferi in vitro and in vivo using a mouse model are continuing. Both monoclonal and monospecific polyclonal antibodies are being developed to help define antigenically variable determinants.